1. Field of the Invention
This invention relates to a gas ventilatory system for ventilating a patient.
2. Description of Related Art
The present invention relates to a ventilatory system for ventilating a patient, comprising a ventilator provided with at least one first inlet for supplying a predetermined first gas mixture to the ventilator, at least one outlet for supplying a controlled amount of the predetermined first gas mixture to the patient during inspiration intervals, the ventilatory system furthermore having a gas administration module connected either downstream or upstream from the ventilator and arranged for administering a predetermined amount of an additional gas to the patient, the gas administration module being provided with a predetermined number of valve means which are connected in parallel and can each be independently switched between a closed state and an open state for supplying a predetermined amount of the additional gas to the patient, each of the valve means being provided with a series connection of a throttle valve with a throttle valve inlet and outlet and a controlled valve with a controlled valve inlet and outlet, the controlled valve being switchable between an open and closed state with a predetermined frequency, the throttle valve outlet being connected to the controlled valve inlet such that there is a passage with a volume V between the throttle valve and the controlled valve.
Such a system is known to the inventors from the firms Drager and Nodomo.
GB-A-2,016,279 discloses a rate of flow controlled automatic medical breathing apparatus. The apparatus comprises a first controlled valve and a second controlled valve downstream from the first controlled valve. Between the first and second valves there is a lone for the flow of a predetermined gas. This line has a side connection to pressure regulating means which generate a control signal for the first valve when the pressure within the line exceeds a predetermined level such that the pressure within the line remains substantially constant.
U.S. Pat. No. 5,471,977 discloses an apparatus for the control of an extremely small additional gas flow to a respiratory gas flow. The apparatus is provided with valves and flow meters for the supply of air and oxygen, and an additional valve and flow meter for the supply of an additional amount of gas like N.sub.2 and NO. The additional valve is controlled independently from the other valves.
The system according to the invention might be used for administering nitric oxide to a patient. Nitric oxide (NO) is a gas with potent vasodilatory properties. Recently, NO is found to have a beneficial effect on oxygen exchange in the lungs of patients with the adult respiratory distress syndrom (ARDS), a serious lung disease that, if untreated, leads to death due to lack of oxygen. ARDS may for instance occur after sepsis (i.e. bacteria or their products in the blood stream), multiple fractures or inhalation of toxic gases. All ARDS patients have to be artificially ventilated in the usual way. Administration of correctly measured NO to these ventilated patients would improve this treatment substantially. Several industrial prototypes have been developed. However, these prototypes suffer from serious problems. Factors causing these problems include:
1. NO reacts with oxygen and produces NO2 and other noxious NOx products. This NOx generation depends on oxygen concentration, time, pressure, temperature and relative humidity and occurs within seconds. NO is therefore stored in N2 usually in concentrations of 100 or 1000 ppm. Because most of the prototypes mix NO with oxygen upstream from the prototypes ventilator, the contact time is relatively long. The known downstream systems operate with more than one gascylinder, these gascylinders differ in NO concentration to cover the relevant clinical range.
2. The addition of NO to O2 decreases the amount of oxygen that can be delivered as large amounts of N2 are supplied together with the NO.
The first problem mentioned above would be solved by adding NO downstream from the ventilator, however, this is impossible with the present designs, without changing the gascylinder. The second problem mentioned above would require higher NO concentrations within the N2 of up to 5000 ppm, thereby improving the NO/N2 ratio. However, this would require greater precision of the NO administration system to a patient.
The ventilatory system according to the prior art lacks sufficient accuracy. The inventors of the present invention has found that this is due to the following problem. The NO module has several parallel valve means, each constituted by a series connection of a throttle valve and a controlled valve. Between the throttle valve and the controlled valve the NO passes through a passage with a relatively large volume. When the controlled valve is closed, still, some NO will flow into the passage thus increasing the pressure. If the controlled valve remains closed, the NO gas will flow through the throttle valve until the pressure within the passage equals the pressure of its inlet. However, when the passage is relatively large the time for neutralizing this interfering effect is relatively long. Actually, no standardization has taken place within the time period associated with the maximum operating frequency of the controlled valve. Thus, when the controlled valve opens again it may happen that the pressure within the passage is not well defined, resulting in an undefined amount of NO administration.